The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile, etc., and specifically relates to such an image forming apparatus that applies heat and pressure onto an unfixed toner image formed on a transfer material by means of a pair of pressing rollers so as to fix the unfixed toner image on the transfer material.
In the image forming apparatus that conducts the image forming operation by employing the electro-photographic method, the toner image formed on an image bearing member (photoreceptor drum) or an intermediate transfer member is transferred onto the transfer material (hereinafter, also referred to as a recording medium or a paper), and the toner image (unfixed toner image) residing on the transfer material is fixed by fixing unit.
The roller fixing method could be cited as a fixing method. According to the roller fixing method, the transfer material, which holds the unfixed toner image on it, is conveyed to a pressure nip section that is formed by a pair of pressing rollers, one of which press-contacts the other one incorporating a heater in it, so as to fix the unfixed toner image on the transfer material by applying heat and pressure at the pressure nip section. Further, the belt fixing method could be cited as another fixing method. According to the belt fixing method, the pressure nip section is formed by a support pressing roller, which is disposed inside a fixing belt so as to support the fixing belt, and an external pressing roller, which press-contacts the fixing belt while placing the fixing belt between the support pressing roller and the external pressing roller, so that the unfixed toner image, residing on the transfer material, faces the heated fixing belt to achieve the fixing operation.
In either the roller fixing method or the belt fixing method, the fixing operation is achieved at the pressure nip section formed by the pair of pressing rollers, which are in a state of press-contacting each other. In this operation, to improve the fixing efficiency, a pressing roller, having a rubber elastic property, is employed, since it is effective for this purpose to lengthen a length of the pressure nip section in a paper conveying direction.
With respect to the structure between the pair of pressing rollers, extensive studies have been carried out.
To prevent the transfer material from wrinkling when passing through the pressure nip section, there has been frequently proposed such a countermeasure that the shape of the pair of pressing rollers should be modified to a certain shape from the conventional cylindrical shape (for instance, set forth in Patent Documents 1-4). In any one of such the proposals, however, there are no descriptions in regard to durability and lifetime of the pressing roller. Instead, there are merely indicated in the drawings such schematic diagrams that show shapes of the pair of pressing rollers, both lengths of which are the same in a rotating-axial direction of them.
Further, with respect to the fixing roller made of a cylindrical glass base, outer surface of which is coated with the elastic layer, to relieve the stress concentration occurring at the end portion of the elastic layer and the load concentration incurred onto the glass base, Patent Document 5 sets forth such a countermeasure that the thickness of the elastic layer is made to gradually decrease in a direction toward the end portion of the elastic layer.
[Patent Document 1]                Tokkai 2004-61700 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkai 2001-265146 (Japanese Non-Examined Patent Publication)        
[Patent Document 3]                Tokkai 2001-215833 (Japanese Non-Examined Patent Publication)        
[Patent Document 4]                Tokkaihei 11-242403 (Japanese Non-Examined Patent Publication)        
[Patent Document 5]                Tokkai 2001-134123 (Japanese Non-Examined Patent Publication)        
As for the pair of pressing rollers on each of which the rubber elastic layer is applied, both pressing rollers are heated up to about 180° C., and the pair of pressing rollers forms the pressure nip section caused by the pressing force generated between them, and further, at least one of the pressing rollers is driven to rotate, in order to conduct the fixing operation for the transfer material, which holds the unfixed toner image on it, either directly or through the fixing belt placed between the pressing rollers. Although a heat resistant resin material is employed for the elastic layer and the elastic layer is formed in a cylindrical shape on the metal axis through a molding process, the repeated load incurred during a long operating term would cause a fatigue of the elastic layer, resulting in partial cracks or a failure of the elastic layer. Accordingly, it becomes difficult to create a uniform pressing state at the pressure nip section for a long time.